1. Field of the Invention
This invention generally relates to acoustic imaging systems and, more particularly, to focused, acoustic scanning apparatus.
2. Description of the Prior Art
In the past the technology of acoustic scanning has been applied to the non-destructive testing of materials, bio-medical diagnosis and underwater detection. In the field of material testing, acoustic scanning has been used to nondestructively detect such defects as manufacturing flaws, surface cracking and internal material fractures, inclusions, debonded areas and other defects. In the area of bio-medical diagnosis acoustic scanning has been used for such purposes as non-intrusive surveys of internal human organs and biological specimens to produce visual images of these organs or specimens for analysis. Underwater imaging has included scanned sonar systems for investigating submerged objects.
Most of these applications have been carried out using a single ultrasonic transducer mechanically translated across the field of investigation. Recently, however, electronically scanned systems using large arrays of discreet transducers and scanned by a plurality of control circuits have been developed.
In addition, there have also been developed several systems for processing the signals used in acoustic scanning systems. One such system uses a chirp generator that provides an RF tone pulse of constant amplitude whose instantaneous frequency increases linearly with time across the width of the pulse. Since World War II chirp generators have been used extensively in the field of radar and electromagnetic scanning. A second component of such signal processing systems is a pulse compression filter. A pulse compression filter is a frequency sensitive circuit element that retards the leading edge of an input pulse so that the trailing edge of the pulse can catch up and become superimposed on the leading edge at the output of the filter.
Chirp generators and pulse compression filters are further explained in the text, "Radar Signals" by Charles E. Cook and Marvin Bernfeld, Academic Press, New York, 1967.
Heretofore, the major problem in developing acoustic scanning devices has been in fabricating units to difficult tolerances that are rugged and not easily susceptible to damage from mechanical shock. Up until this time acoustic scanning equipment has been either quite fragile and easily broken or required close operating tolerances that were difficult to fabricate and easily disturbed.
Another problem in developing acoustic scanning systems has been the inability to develop designs that can operate effectively throughout the usable acoustic spectrum. Most of the prior systems are operable in the lower frequency portions of the usable acoustic spectrum, usually below 10 MHz.
One further problem with signal processing systems has been that most pulse compression filters are neither tunable nor have the capacity to vary the slope and pulse width of the chirp. Most commercially available generators and pulse compression filter combinations are designed to operate with only predetermined wave forms and must be carefully matched for their intended use.